The Laser Megajoule (LMJ) is a French large scale laser facility dedicated to inertial fusion and plasma physics research. LMJ front-ends are based on fiber laser technology at nanojoule range [1]. Scaling the energy of those fiber seeders to the millijoule range is a way to upgrade LMJ’s front ends architecture and could also be used as seeder for lasers for ELI project for example. However, required performances are so restrictive (optical-signal-to-noise ratio higher than 50 dB, temporally-shaped nanosecond pulses and spatial single-mode top-hat beam output) that such fiber systems are very tricky to build.
High-energy fiber amplifiers
In 2015, we have demonstrated, an all-fiber MOPA prototype able to produce a millijoule seeder, but unfortunately not 100% conform for all LMJ’s performances. A major difficulty was to manage the frequency modulation used to avoid stimulated Brillouin scattering, to amplitude modulation (FM-AM) conversion, this limits the energy at 170µJ.
For upgrading the energy to the millijoule range, it’s necessary to use an amplifier with a larger core fiber. However, this fiber must still be flexible; polarization maintaining and exhibit a strictly single-mode behaviour. We are thus developing a new amplifier architecture based on an Yb-doped tapered fiber: its core diameter is from a narrow input to a wide output (MFD 8 to 26 µm). A S² measurement on a 2,5m long tapered fiber rolled-up on 22 cm diameter confirmed that this original geometry allows obtaining strictly single-mode behaviour. In a 1 kHz repetition rate regime, we already obtain 750 µJ pulses, and we are on the way to mJ, respecting LMJ performances.
Beam delivery
In LMJ architecture the distance between the nanojoule fiber seeder and the amplifier stages is about 16 m. Beam delivery is achieved with a standard PM fiber, such a solution is no longer achievable with hundreds of kilowatt peak powers. An efficient way to minimize nonlinear effects is to use hollow-core (HC) fibers. The comparison between the different fibers will be presented in the conference.
Fiber spatial beam shaping
Spatial beam shaping (top-hat profile) is mandatory to optimize the energy extraction in free-space amplifier. It would be very interesting to obtain a flat-top beam in an all-fiber way. Accordingly, we have design and realize a large mode area single-mode top-hat fiber able to deliver a coherent top-hat beam. This fiber, with larger MFD adapted to mJ pulse, will be implemented to perform the spatial beam shaping from coherent Gaussian profile to coherent top-hat intensity profile in the mJ range. In conclusion, we will present an all-fiber MOPA built to fulfil stringent requirements for large scale laser facility seeding. We have already achieved 750 µJ with 10 ns square pulses. Transport of high peak power pulses over 17 m in a hollow-core fiber has been achieved and points out FM to AM conversion management issues. Moreover, spatial beam shaping is obtained by using specifically designed single-mode fibers. Various optimizations are currently under progress and will be presented.

The Laser megajoule (LMJ) is a French large scale laser facility dedicated to inertial fusion research. Its front-ends are based on fiber laser technology and generate highly controlled beams in the nanojoule range. Scaling the energy of those fiber seeders to the millijoule range is a way explored to upgrade LMJ’s architecture.
We report on a fully integrated narrow line-width all-fiber MOPA prototype at 1053 nm designed to meet stringent requirements of large-scale laser facilities seeding. We achieve 750 µJ temporally-shaped pulses of few nanoseconds at 1 kHz. Thanks to its original longitudinal geometry and its wide output core (26µm MFD), the Yb-doped tapered fiber used in the power amplifier stage ensures a single-mode operation and negligible spectro-temporal distortions. The transport of 30 kW peak power pulses (from tapered fiber) in a 17 m long large mode area (39µm) hollow-core (HC) fiber is presented and points out frequency modulation to amplitude modulation conversion management issues. A S² measurement of this fiber allows to attribute this conversion to a slightly multimode behavior (< 13dB of extinction between the fundamental mode and higher order modes). Other HC fibers exhibiting a really single-mode behavior (<20 dB) have been tested and the comparison will be presented in the conference. Finally, fiber spatial beam shaping from coherent Gaussian beam to coherent top-hat intensity profile beam in the mJ range with a specifically designed and fabricated fiber will also be presented.

In large scale laser facility dedicated to laser-matter interaction including inertial confinement fusion, such as LMJ or NIF, high-energy main amplifier is injected by a laser source in which the beam parameters must be controlled. For many years, the CEA has developed nano-joule pulses all-fiber front end sources, based on the telecommunications fiber optics technologies. Thanks to these technologies, we have been able to precisely control temporal shaping and phase-modulated pulse. Nowadays, fiber lasers are able to deliver very high power beams and high energy pulses for industrial needs (laser marking, welding,…). Therefore, we have currently developed new nanosecond pulses fibered amplifiers able to increase output pulse energy up to the mJ level. These amplifiers are based on flexible fibers and not on rod type. This allows us to achieve a compact source. Nevertheless the intensity profile of theses fibers usually has a Gaussian shape. To be compatible with main amplifier section injection, the Gaussian intensity profile must then be transformed into ‘top-hat’ profile. To reach the goal, we have recently developed an elegant and efficient solution based on a single-mode fiber which directly delivers a spatially coherent ‘top-hat’ beam. In the conference, we will present this mJ-class top-hat all-fiber laser system, the results and the industrial prototype which can be used as a front-end of high-power lasers or as a seeder for other types of lasers.

The development of ultra-broadband oscilloscopes is mainly governed by the needs of future telecom networks. But
other applications are requesting the availability of true real-time acquisition oscilloscopes. Systems able to be used in
single-shot operation are of prime interest for Inertial Confinement Fusion (ICF) and for the related R&D for plasma
physics.
We previously demonstrate a single-shot, 100GHz design of an all-optical sampling oscilloscope at 1μm (MULO). This
laboratory system has been improved in stability and compactness to make an all-in-one box prototype. More, by the
addition of an opto-electro-optics (OEO) sub-system at the input, we developed the ability to use this oscilloscope to
analyze an electrical input signal up to 60GHz. This new integrated subset also increases the range of wavelength for
optical input signal, from 300nm up to 2μm. Furthermore, it allows the use of inexpensive opto-electronic components at
telecom wavelength for this system regardless of the signal to be analysed. In parallel with these improvements, by
optimizing the heart of the system, we get a very high sampling rate, up to 500Gs/s and more; this allows considering
much higher bandwidths in the future.
In this talk, we will present latest developments and integration of this system. It will also allow us to give more details
on the innovative OEO sub-system.

In lasers used for inertial confinement fusion (ICF) both temporal and spectral performances have to be controlled with accuracy. As commercial systems do not allow accurate enough measurements, we developed new diagnostics. For spectral measurements, we developed an innovative highly resolving spectrometer. This system allows a 1GHz resolution measure of spectrum in single-shot operation. For temporal shape measurement, we implemented upgrades and go on with the pre-industrial integration of our previous early design1, in an all-in-one box system. This system enables real-time analysis of optical pulse shapes for wavelengths from 300nm up to 2μm. Thanks to an innovative optical-electro-optical (OEO) sub-converter, it is also possible to measure electrical pulses, with 60GHz bandwidth at 500Gs/s and up to 3Ts/s sampling rate and more than 8-bit dynamics range. We developed an all fibered system that allows direct measurement of temporal Dynamic Extinction Ration (DER)3 for pulsed laser in single shot operation. This device could be adapted to several wavelengths and allows achieving a measurement up to 60dB of DER with 1dB accuracy. In brief, we will give an up-to-date description of some recent development in high precision diagnostics applied to LMJ front-end.

LMJ is typical of lasers used for inertial confinement fusion and requires a laser of programmable parameters for
injection into the main amplifier. For several years, the CEA has developed front end fiber sources, based on
telecommunications fiber optics technologies. These sources meet the needs but as the technology evolves we can expect
improved efficiency and reductions in size and cost.
We give an up-to-date description of some present development issues, particularly in the field of temporal shaping with
the use of digital system. The synchronization of such electronics has been challenging however we now obtain system
jitter of less then 7ps rms.
Secondly, we will present recent advance in the use of fiber based pre-comp system to avoid parasitic amplitude
modulation from phase modulation used for spectral broadening.

An original polarization - maintaining Sagnac switch is proposed for use in optical sampling and short pulse measurement applications, in the range of signal wavelengths of interest for Inertial Confinement Fusion. Our design is implemented using highly-nonlinear
photonic-crystal fibres. It enables the search of huge switching contrasts together with very large sampling bandwidths, in relationship with an elevated temporal resolution. A unique
two-pass Sagnac loop is fed with input signal pulses at 1053nm while triggered with pump pulses at 1550nm. Starting from a
single-pass contrast and a temporal resolution in the ranges of 30dB and of a couple of picoseconds, the two-pass architecture provides optical contrasts in excess of 45dB and sub-picosecond gating durations. Thanks to two-pass operation, we can get nearly free from any environmental perturbation. Furthermore the spectral and the temporal clipping features related to switching are analyzed using comprehensive modeling with higher order dispersion effects. The issue of the optimization of the sampling bandwidth is discussed in details by means of the synchronization of the pump return, which involves a sub-picosecond precision. This way, the output energy from the switch can be kept constant and proportional to the signal power, whatever the input pulse width. The sampling bandwidth then extends up to RF frequencies in the range 300-500GHz.

A new theoretical approach for modeling the saturated single pass gain in a three-level fiber amplifier is presented, relevant to the behavior of rare-earth-doped silica fibers. A basic approximation considering the stimulated emission rate Ws(z,ν) as a dimensionless parameter S, independent of the spatial and frequency variables z and ν, allows to obtain analytic expressions for input and output pump, ASE and signal powers inside the fiber core. We show that these expressions only depend on the S parameter, which is determined by solving the photon balance equation, and S is shown to be fully representative of the saturation in the medium. The main result of the model is that the pump repartition P(z) takes a simple analytical form, which can be separated into two parts, below and above the saturated absorption length L0, which is a function of S. The first part 0〈z〈L0 is the saturated absorption where the pump distribution is linear and the second part L0〈z〈L is the non saturated absorption region with an exponentially decreasing pump distribution. Compared to other analytical models with the assumption of an averaged inversion population 〈N2(z)〉, we obtain a good description of the difference between the co- and counter-propagating ASE behaviors accounting for the fiber length and the pumping level. The model, which is well suited to longitudinal pumping, can also describe a side pumped fiber amplifier by simple adjustment of some of the model parameters.

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